The enzyme cytochrome c oxidase or Complex IV EC.9.3.1) is a large transmembrane protein complex found in bacteria and the mitochondrion.It is the last enzyme in the respiratory electron transport chain of mitochondria (or bacteria) located in the mitochondrial (or bacterial) membrane. It receives an electron from each of four cytochrome c molecules and transfers them to one oxygen molecule converting molecular oxygen to twomolecules of water. In the process it binds four protons from the inner aqueous phase to make water and in addition translocates four protons across the membrane helping to establish a transmembrane difference of proton electrochemical potential that the ATP synthase then uses to synthesize ATP.